Search

Comparison study of unsteady Reynolds-averaged Navier-Stokes (URANS) and hybrid RANS-LES models is carried out for predicting the performance of three-straight-bladed vertical axis wind turbine operating within tip speed ratios (TSRs) ranges of (1.44–3.3). The evaluation is focused on power coefficient, moment coefficient and vortex structure generation, growth and transportation predictions. It was found that URANS turbulence modelling is sufficient for averaged power coefficient prediction and specific range of TSR evaluation, and it also benefits from short simulation run time. To further evaluate flow field details and to understand the underlying flow physics such as dynamic stall behaviour, hybrid RANS-LES turbulence modelling is necessary. Comparing between hybrid models adopted, stress-blended eddy simulation based on transition shear-stress transport (TSST) model has achieved an overall better performance, such as a reduction of simulation discrepancy by 50% in low TSRs range compared to TSST turbulence model. In both medium and high TSRs ranges, the modelling discrepancies are less than 3% compared to TSST turbulence models at about 25% extra computational time. Furthermore, in high TSRs range, hybrid RANS-LES models are able to predict the appearance of vortex shedding at high azimuthal angles (θ ≥ 180°) for which URANS models failed to capture.

Purpose The aim of this paper is to assess the ability of a stress-blended eddy simulation (SBES) turbulence model to predict the performance of a three-straight-bladed vertical axis wind turbine (VAWT). The grid sensitivity study is conducted to evaluate the simulation accuracy. Design/methodology/approach The unsteady Reynolds-averaged Navier–Stokes equations are solved using the computational fluid dynamics (CFD) technique. Two types of grid topology around the blades, namely, O-grid (OG) and C-grid (CG) types, are considered for grid sensitivity studies. Findings With regard to the power coefficient (Cp), simulation results have shown significant improvements of predictions using compared to other turbulence models such as the k-e model. The Cp distributions predicted by applying the CG mesh are in good agreement with the experimental data than that by the OG mesh. Research limitations/implications The current study provides some new insights of the use of SBES turbulence model in VAWT CFD simulations. Practical implications The SBES turbulence model can significantly improve the numerical accuracy on predicting the VAWT performance at a lower tip speed ratio (TSR), which other turbulence models cannot achieve. Furthermore, it has less computational demand for the finer grid resolution used in the RANS-Large Eddy Simulation (LES) “transition” zone compared to other hybrid RANS-LES models. Originality/value To authors’ knowledge, this is the first attempt to apply SBES turbulence model to predict VAWT performance resulting for accurate CFD results. The better prediction can increase the credibility of computational evaluation of a new or an improved configuration of VAWT.

Optimisation of Gurney flap (GF) geometries mounted on a three-straight-bladed vertical axis wind turbine (VAWT) is evaluated with computational fluid dynamics (CFD) solution and Taguchi method, considering the blade rotating effect at three tip speed ratios (TSRs) of low, medium and high ranges. A hybrid RANS-LES model applying stress-blended eddy simulation with transition shear-stress transport turbulence model is adopted. Results analysis confirms that GF geometry optimisation needs to consider multiple rotational blades rather than a single stationary one, owing to different optimum GF geometries and their performances at a different range of TSRs. It is found that VAWT with GF can significantly improve the power coefficient at low range TSRs (up to 233.19%), but it decreases with the increase of TSRs ranges (up to 69.94% and 41.36% at medium and high ranges of TSRs, respectively).

Recently, the Gurney Flap (GF) has been used to improve the performance of Horizontal Axis Wind Turbine (HAWT) by enhancing its lift coefficient. Compared to HAWT, research on GF application for Vertical Axis Wind Turbine (VAWT) is very limited. Moreover, most works studied a GF geometry attached to the trailing edge of a stationary airfoil, without considering the rotating effect experienced by VAWT. For this reason, a three-straight-bladed VAWT rotating blade with GF is studied by transient RANS simulation together with a stress-blended eddy simulation (SBES) turbulence model to investigate the GF height effect and the flow characteristics near the blade trailing edge. Results have shown that by introducing the blade rotating, an optimum GF height is found to be 3% of the blade chord, slightly higher than 2% chord in a stationary airfoil case. In addition, the presence of GF can delay deep stall of VAWT blades, thus eliminating negative instantaneous moment coefficient and improving the turbine performance.

Small signal stability represents the reliability of generator for transferring electrical energy to the consumers. The stress of the generator increases proportionally with the increasing number of load demand as well as the uncertainty characteristic of the load demand. This condition makes the small signal stability performance of power system become vulnerable. This problem can be handled using power system stabilizer (PSS) which is installed in the excitation system. However, PSS alone is not enough to deal with the uncertainty of load issue because PSS supplies only an additional signal without providing extra active power to the grid. Hence, utilizing capacitor energy storage (CES) may solve the load demand and uncertainty issues. This paper proposes a coordination between CES and PSS to mitigate oscillatory behavior of the power system. Moreover, bat algorithm is used as an optimization method for designing the coordinated controller between CES and PSS. In order to assess the proposed method, a multi-machine two-area power system is applied as the test system. Eigenvalue, damping ratio, and time domain simulations are performed to examine the significant results of the proposed method. From the simulation, it is found that the present proposal is able to mitigate the oscillatory behavior of the power system by increasing damping performance from 4.9% to 59.9%.

Numerical simulations for relatively thick airfoils are carried out in the present studies. An attempt to improve the accuracy of the numerical predictions is done by adjusting the turbulent viscosity of the eddy-viscosity Menter Shear-Stress-Transport (SST) model. The modification involves the addition of a damping factor on the wall-bounded flows incorporating the ratio of the turbulent kinetic energy to its specific dissipation rate for separation detection. The results are compared with available experimental data and CFD simulations using the original Menter SST model. The present model improves the lift polar prediction even though the stall angle is still overestimated. The improvement is caused by the better prediction of separated flow under a strong adverse pressure gradient. The results show that the Reynolds stresses are damped near the wall causing variation of the logarithmic velocity profiles.